1. Field of the Invention
The present invention relates generally to diode pumped lasers and amplifiers and more specifically it relates to diode pumped vibrational-rotational transition gas lasers and amplifiers in which the active medium comprises a mixture of alkali vapor, selected hetero-nuclear molecular gas, and one or more buffer gases.
2. Description of Related Art
Mid-infrared vibrational-rotational transition, hetero-nuclear molecular lasers have been known for years. Typically, these lasers are energized using either chemical reactions (as in the HF, DF, HBr, CO, etc. lasers), or electrical discharges (as in the CO lasers).
The main problem with chemically pumping these lasers is the need to provide highly reactive, potentially dangerous chemical precursors to generate excited laser molecules. In the case of military use of such lasers, it is also necessary to establish a separate costly logistics supply chain of chemical precursors in the field. The main problem with electrical discharge pumping of CO lasers is that such lasers typically produce radiation at output wavelengths at approximately greater than 5.6 microns (from higher-lying vibrational levels), radiation which does not transmit through the atmosphere with low loss (i.e., as does radiation in the 4.6–5.3 microns region). Thus, there has been a long-standing need to replace the chemical or electrical discharge excitation sources with more effective and practical excitation sources to realize practical and useful mid-infrared hetero-nuclear molecular lasers.
One approach to solving this problem is the use of a laser pump source to directly excite the vibrational levels of the laser active hetero-nuclear molecules by direct absorption of pump radiation. To efficiently couple laser pump energy into the hetero-nuclear molecules, whose ro-vibrational transitions are characterized by quite narrow spectral widths, it is necessary to use spectrally narrowed pump laser sources [ref. 1–4, and references therein]. While the efficacy of optical pumping has been validated in terms of the physics of laser action, the realization of practical optical pumping of mid-infrared hetero-nuclear molecular lasers cannot be achieved in this manner because the efficiencies of laser pump sources are necessarily depressed because of the need to line-narrow these sources and to set their output wavelengths precisely to the narrow absorption features of the molecular laser gas. Thus, an intrinsically more efficient optical pumping scheme is needed.
Krupke [ref. 5] has patented the concept of efficient, diode-pumped alkali lasers operating on their respective D1 resonance transitions, utilizing low-cost conventional broad-area semiconductor laser diodes and diode arrays with output spectral bandwidths of a nm or more to pump the collision-broadened, spectrally-homogeneous D2 transitions. Notwithstanding the relatively narrow spectral widths of the collision-broadened D2 transitions (typically <0.2 nm at 10 atm of helium buffer gas), efficient absorption of the output radiation of such laser diode sources is achieved by end-pumping the laser gain cell, whose atomic density and length allows for efficient wing-pumping of the alkali atoms. Model calculations, and preliminary laboratory experiments [ref. 6] using a titanium sapphire laser as a surrogate pump source, indicate that pump radiation absorption efficiencies may exceed 80%, and optical-optical laser efficiencies in the range from 50–70% can be attained.